1. Field of the Invention
The invention pertains to the field of shielded cables for use in high speed data transmissions and associated cable retention mechanisms. More specifically, ground receptacles are used in mounting the cables to electrical housings or chassis in a manner that minimizes electromagnetic interference (EMI).
2. Discussion of the Related Art
Cables for use in transmitting electronic signals are often provided with shielding in the form of foil, wire mesh or screen material that surrounds one or more central insulated leads. One common example of this type of cable is the coaxial cable that is used to carry television or data transmissions; however, in computer applications, there are often a plurality of data transfer lines in the form of a cable bundle. The shielding itself is usually covered with an external layer of insulation or a protective outer layer. The shielding protects the signal that is being transmitted on the central lead from ambient electromagnetic disturbances. The shielding also limits the amount of electromagnetic disturbance that is transmitted outwardly from the central lead. For these reasons, shielded cables are increasingly utilized in densely packed arrays of electrical equipment.
A variety of connectors are used to secure and interconnect these cables. Typically, bulkhead connectors, which connect the shielding to a ground proximate the terminus of the cable, are used for shielded cables. Nevertheless, it is not always desirable or practical to connect the cable to ground solely at its terminus. For example, U.S. Pat. No. 5,975,953 to Peterson describes the difficulties and special considerations that are involved when connecting electromagnetic interference (EMI) shielded cables directly to an input/output (I/O) card and having to shunt the ground path through the I/O card.
Further, in the case of bulkhead connectors, a continuing problem exists with securing the cables against unwanted motion that can, for example, cause signal degradation by torsional or translational motion of the cable. Prior systems are unable to secure the cable against unwanted motion while providing a ground for the EMI shielding. Separate structures, such as a rubber grommet that is separate from the bulkhead connector are often used to limit such motion, but constitute poor electrical conductors and may facilitate EMI leaks form an otherwise closed EMI housing.
Regulatory agencies are promulgating ever stricter regulations that increasingly limit the amount of EMI which electronic equipment may generate. Additionally, stricter EMI limits are necessarily imposed by the practicalities of operating computer and telecommunications systems at increasingly faster rates of data transmission. Whenever a cable passes through the wall of an electrical housing or chassis, the opening may provide an unacceptable EMI leak. Increasingly, it is necessary to seal openings electrical housings, in order to prevent EMI leaks. At the same time, it is useful to be able establish EMI seals at multiple optional locations on a single cable construction, to reduce the number of unique cable constructions which would otherwise have to be manufactured and stocked, and to configure these as needed at the time of installation.
The present invention overcomes the problems that are outlined above by providing a shielded cable assembly for use in conveying data signals through a chassis wall with a substantially complete EMI seal.
The shielded cable comprises at least one data transfer line, which may also be part of a cable bundle. The data transfer line typically includes an electrically conductive lead for use in transmitting the signal; however, use of a data transfer line communicating optical signals is also contemplated, as are combinations of electrical and optical data transfer lines in a single cable. Electromagnetic shielding surrounds the data transfer line. The shielded cable contains a first hardpoint comprising a conduit positioned around the data transfer line over an interval at a preselected location for use in passing through the chassis wall. The conduit defines an interior passageway and has an exterior side. The data transfer line passes through the interior passageway of the conduit, and the electromagnetic shielding surrounds the exterior side of the conduit. Thus, when a clamp assembly is placed around the shielding over the conduit, a superior mechanically secure ground connection is established by compressive forces that are exerted between the exterior side of the conduit and the clamp assembly.
Various other components are optionally provided in the shielded cable. For example, a protective covering may surround the data transfer line at a position within the electromagnetic shielding. The protective covering is absent over a section of the interval over the conduit. The conduit may have a first end inserted within the protective covering, a second end inserted within the protective covering, and a middle section comprising the area where the protective covering is absent. An electrically insulative material, such as adhesive-backed electrical tape, may be positioned around the conduit and within the electromagnetic shielding to secure the conduit. An outer sleeve may be placed around the data transfer line and the electromagnetic shielding, but a portion of the outer sleeve must be removed over the section of the conduit where the protective covering is also absent, in order to facilitate access to the electromagnetic shielding for purposes of establishing a ground connection.
In some embodiments, a selectively detachable tube covers electromagnetic shielding over the section of the conduit where the outer sleeve is absent. The selectively detachable tube may comprise a tear-away material having a pattern of holes that are spaced apart at distances which facilitate removal of the selectively detachable tube in predetermined lengths. A shrink-wrap layer may surround at least one end of the conduit. The shrink wrap layer overlaps the outer sleeve at a position outboard of the outer sleeve and underlap the selectively detachable tube inboard of the selectively detachable tube for containment of metal fragment from the electromagnetic shielding. The provision of a selectively detachable tube expedites custom installation procedures by affording quick access to the electromagnetic shielding, which is exposed by simply peeling away the selectively detachable tube.
The data transfer line may comprise a first end connected to a first cable coupling, a second end connected to a second cable coupling. A plurality of hardpoints may be provided including a hardpoint at any interval location where the shielded cable could pass through an electrical housing or chassis.
A clamp assembly, such as a separable block, may be used to retain the shielded cable on the electrical housing or chassis and to establish a grounded connection between the electromagnetic shielding and the chassis by virtue of compressive forces over the interval between the clamp assembly and the conduit. The conduit is sufficiently rigid to withstand the compressive forces from the clamp assembly while protecting the data transfer line from damage that would, otherwise, possibly inure to the data transfer line from the compressive forces. As used herein, the term xe2x80x9cseparable blockxe2x80x9d encompasses a clamping structure that is formed of two or more pieces, as well as structures where these pieces are joined by a hinge or pivot, for example, in a bivalve configuration.